Coenzyme Q10 (CoQ10) is a small lipophillic molecule composed of a benzoquinone ring and a hydrophobic isoprenoid tail and is present in virtually all cell membranes. In the mitochondrial respiratory chain, CoQ10 is vital for the transport of electrons from complex I (NADH-ubiquinone oxidoreductase) and complex II (succinate-ubiquinone oxidoreductase) to complex III (ubiquinol-cytochrome c reductase). It is also an antioxidant, membrane stabilizer, and modulator of apoptosis. Human CoQ10-deficiency has been associated with four clinical phenotypes: an encephalomyopathy, infantile encephalomyopathy with renal dysfunction, cerebellar ataxia, and pure myopathy. We have identified 38 patients from 33 families with CoQ10-deficiency. Patients with all forms of CoQ10-deficiency have improved with oral supplementation, therefore recognition of this treatable genetic condition is important. Last year, we reported the first mutations responsible for primary CoQ10-deficiency; in two siblings with the infantile form of CoQ10 deficiency, we identified a homozygous missense mutation in the COQ2 gene which encodes para-hydroxybenzoate-polyprenyl transferase, the enzyme responsible for the condensation of the isoprenoid side chains to the benzoquinone ring. In another infant with fatal Leigh syndrome and nephropathy, we identified compound heterozygous mutations in the PDSS2 gene encoding subunit 2 of decaprenyl diphosphate synthase. In addition, in a family with four individuals with cerebellar ataxia and CoQ10 deficiency, we identified a pathogenic mutation in the APTX gene, which encodes a protein involved in single-strand break repair. Thus, our studies have revealed mutations in CoQ10 biosynthetic genes causing primary CoQ10 deficiency and an APTX mutation as a cause of secondary CoQ10 deficiency. Not surprisingly, CoQ10 deficiency causes defects of respiratory chain activities (reduced activities of complexes I+III and II+III). The relative importance of respiratory chain defects, ROS production, and apoptosis in the pathogenesis of CoQ10-deficiency is unknown. Based on our preliminary studies, we hypothesize that: 1) genotypic heterogenity contributes to the phenotypic variability of CoQ10-deficiency; 2) levels of oxidative stress, respiratory chain deficiency, and apoptosis differ depending on the genetic cause and severity of CoQ10-deficiency; and 3) CoQ10 and its analogs have variable efficacy in CoQ10-deficiency. To test these hypotheses, we propose the following four specific aims. Aim 1: To identify novel genetic causes of CoQ10 deficiency. Aim 2: To characterize the pathogenic mechanisms of CoQ10-deficiency in cultured cells and rescue cellular dysfunction in CoQ10-deficient cells using CoQ10 and its analogs. Aim 3: To generate and assess a mouse model of CoQ deficiency. Aim 4: In collaboration with Professor Plcido Navas, University of Sevilla, Spain, we will characterize CoQ10 biosynthetic genes and mutations in yeast.